Effect of humidity and oxygen on friction, wear and durability of a polymer-bonded molybdenum disulfide (MoS2)-based dry film lubricant (DFL) coating system in large amplitude fretting

Investigations into effects of humidity and oxygen on a polymer-bonded MoS2-based dry film lubricant (DFL) coating, on grit-blasted Ti–6Al–4V using a cylinder-on-flat configuration, are reported. Sustained low friction and low wear occurred in both low-humidity air and low-humidity argon, producing significant extension of fretting wear lifetimes. A cracked-paved structure was observed in low-humidity air which was not observed in low-humidity argon. The Mo-DFL coatings were observed to be highly sensitive to humidity. As humidity increased, a notable rise of friction and wear ensued which reduced the system lifetime. Different friction evolution was observed in high-humidity air and high-humidity argon before failure, with a short-lived but established low friction phase in high-humidity air and a consistent high friction phase in high-humidity argon. Although oxidation of the MoS2 to MoO3 is a key process of degradation of DFLs of this type, this oxidation reaction does not operate in the absence of humidity. •Experimental fretting tests carried out on MoS2 loaded polymer DFL systems in a range of environmental conditions.•Fretting is very sensitive to RH, low RH fretting lifetimes were an order of magnitude larger than for atmospheric conditions.•Oxygen appears to contribute to the formation of a low friction phase within the evolution of friction.